Riddled with sores, his lips locked on a crack pipe, the “sub-basement”-dwelling subject of this cult-rock doc initially seems plucked from an episode of “Intervention,” or maybe “Hoarders.” But back in the ’70s, Bobby Liebling was the flamboyant, Dali-esque frontman of “doom-metal” group Pentagram. “Why doesn’t everyone talk about them?” wonders Sean “Pellet” Pelletier, Liebling’s long-suffering manager and a member of the band’s small but fervent fan base.

Lots of reasons, it turns out, most of them having to do with Liebling’s self-destruction. All bulging eyes and terrible teeth, he’s 53 going on 90; he’d make a stellar anti-drugs poster boy.

Long ago, his “street Black Sabbath” band was poised for success, but heroin, missed gigs, bandmate fights and near-comical delusions of grandeur landed Liebling back home with his parents, picking at imaginary parasites and incoherently reminiscing. It’s horrifying, and you can’t look away.

The redemptive third act of Don Argott and Demian Fenton’s (“The Art of the Steal”) film, which involves a Webster Hall reunion and the love of a good — if cringe-inducingly young — woman, seems as much a surprise to Liebling as it is to us.